Recently there has been widespread acceptance of wireless applications, which has triggered a huge demand for radio spectrum. After many years of spectrum assignment, there exists a heavily crowded spectrum where most spectrum bands have already been assigned to licensed (primary) users, the current spectrum assignment, however, presents a low spectrum usage, improving the spectrum utilization is hence raising an urgent question.
Among the recent technologies, Joseph Mitola, the adviser of MITRE Co & Doctor of Sweden Imperial Technology University, and professor GERALD Q MAGUIRE, JR, put forward the cognitive radio technology in the IEEE Personal Communications in August 1999. This so called Cognitive Radio technology reasons the radio spectrum action controlling rule (such as radio frequency band, air interface, protocol and space & time model etc) by the model-based method in the radio field, with the realizing language through radio knowledge (RKRL), express the knowledge of radio rule, equipment, software module, wave propagation property, network, user demand and application scene, and thus increase the personal business flexibility to meet users requirements better with the software radio technology. Its core ideal is to sense intellectually which part of spectrum bands is being used or not, and the cognitive radio system may use the same spectrum band to communicate temporary in case of sensing out that a certain spectrum bands aren't be used. In time of a secondary user using the licensed spectrum source in “borrow” way, his communication mustn't affect the other primary user's communication. Secondary user, therefore, should employ the non-used spectrum band in accordance with certain rule. Among the cognitive radio technology, this kind of rule is loaded to the communication terminal with a certain form understood by machine (such as XML language). Owing to the fact that the rule could be adjusted according to the spectrum usage, communication load and distribution etc, the spectrum administrator could control the valuable spectrum source more flexible by these rules.
The cognitive radio technology put forwarded as a newly conception has an even widen coverage. The basic definition of cognitive radio is given by that the cognitive radio is a kind of intellectually communication system sensible to outside communication environment. The cognitive radio system senses the outside environment changement continuously by study, and adapts to the environment changement through adaptive adjusting the communication mechanism within itself. This adaptive adjusting is to improve the system stabilization on one side, and increase the spectrum source output coefficient on the other side.
Although much work has been done in cognitive networks dynamic distribution, most previous work considers maximizing the throughput of secondary users as one of the most important design criteria. As a consequence, other qualities of service (QoS) measures for secondary users, such as distortion for multimedia transmission, are mostly ignored in the literature. However, such as video, recent work in cross-layer design shows that maximizing throughput does not necessarily benefit the quality of service at application layer for some multimedia applications. From a users' point of view, the quality of service for cognitive radio is surely poor than that of other general radio access and is more important than that at other layers.
Most of the wireless local area network (WLAN) technology and wireless personal area network (WPAN) technology use the unlicensed spectrum bands, among which, the multimedia applications such as video conferencing got a rapid development. Lossy video compression standard such as MPEG4 and H264 exploit the spatial redundancy to reduce the required bandwidth to transmit video. Compressed video comprises of intra- and inter-coded frames. The intra refresh rate is an important application layer parameter. Adaptively adjusting the intra-refresh rate for online video encoding applications, can combat the time varying wireless channels available to secondary users to improve the service quality to secondary users for transmission over cognitive radio networks.